In the pharmaceutical and chemical industries, liquid substances are commonly micro-dosed, i.e. dosed in the range of micro-liters. For example, in some medical applications liquid drugs are administered to a patient's body via micro-dosing.
Patients frequently utilize portable computer controlled micro-dosing devices to administer medication for the treatment of diabetes and in certain pain treatments. The micro-dosing devices can continuously administer medication to a patient as needed throughout the day depending on the demands of the patient. Additionally, the micro-dosing devices may include a drug reservoir which contains a several days supply of medication. Such micro-dosing devices may also be capable of administering the medication subcutaneously. For example, many insulin pumps contain a supply of medication that is administered to the patient's body according to a predetermined daily profile via an infusion cannula.
Since the continuous administration of medication takes place during daily activities, it is desirable to conceal the micro-dosing devices while they are being carried by the patient. Additionally, it is desirable for the micro-dosing devices to be small in order to be concealed by clothing when worn at the patient's body and for wearing comfort.
The outer dimensions of many portable micro-dosing devices are commonly influenced by the space requirements of the liquid reservoir and of a drive unit of the device. One type of micro-dosing device employs a disposable vial with a displacement piston as a liquid reservoir and a drive unit, respectively. A liquid drug is stored in the disposable vial and dispensed from the disposable vial by moving the displacement piston towards the discharge end of the disposable vial.
U.S. Pat. No. 6,248,093 B1 describes embodiments of micro-dosing devices with drive units and reservoirs in various arrangements. In some embodiments the drive unit comprises a drive motor arranged beside the reservoir. Such micro-dosing devices include a transmission gear, which causes additional costs and friction. Furthermore, the transmission gear is prone to wear and tear, and increases the space requirements of the micro-dosing device.
In another embodiment of U.S. Pat. No. 6,248,093 B1, the micro-dosing device comprises a drive motor arranged in line with a liquid reservoir and a threaded appendix of a reservoir piston. The reservoir piston engages a gear wheel of the drive motor with its thread and is thereby linearly displaceable by the drive motor for pushing the piston of the reservoir. The piston is pushed in a retracted position that axially extends substantially over the entire length of the drive motor at the outer periphery thereof, resulting in a relative compact design. However, the threaded appendix at its side facing away from the gear wheel of the drive motor needs to be supported, in which a backlash free arrangement is necessary in order to ensure precise dosing, In such an arrangement, considerable friction and asymmetric transmission of forces into the piston are generated. As a result, imprecision is caused by the tilting of the piston and the deformation of parts under load.
In a further embodiment of U.S. Pat. No. 6,248,093 B1, the micro-dosing device comprises a drive unit having a drive motor with a reduction gear, both arranged in line with a reservoir. A threaded sleeve engages a gear wheel of the drive unit and is thereby linearly displaceable by the drive unit for actuating a piston of the reservoir, in a retracted position that completely circumferentially surrounds the reduction gear but not the drive motor. The length of the motor adds to the length of the micro-dosing device and the outer diameter of the threaded bushing is correlated with the minimum inner diameter of the reservoir. Thus, even though from a mechanical point of view this design avoids the disadvantages of the before mentioned embodiments, it is unsatisfactory as the length and the thickness of the micro-dosing device are increased.
Hence, it is a desire to provide a method of and a portable device for micro-dosing a liquid, which at least partially avoid the disadvantages of the above mentioned prior art.